Mitogen activated protein (MAP) kinases are a large and diverse group of Ser/Thr kinases separated into three major subgroups, which include the extracellular signal regulated kinases (ERKs), the c-Jun N-terminal kinases (JNKs)/stress-activated protein kinases (JNKs) and p38/reactivating kinases (RK). The ERKs are activated by mitogens and growth factors, whereas the JNKs/SAPKs and p38/RK are activated by bacterial lipopolysaccharide (LPS, interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α) and cellular stresses such as heat shock, osmotic shock, or UV damage. Exposure of cells to these factors results in the increased production of proinflammatory cytokines. Analysis of a specific inhibitor of p38 MAP kinase, SB203580, reveals that it inhibits LPS-induced cytokine synthesis in human monocytes, thus indicating that p38 is the MAP kinase responsible for stress-induced cytokine production (1). SB203580 also prevents the activation of MAP kinase activated protein kinase 2 (MK2), suggesting that this kinase is activated by P38 (2).
Mice engineered to be homozygously-deficient in MK2 show a reduction in TNF-α, interferon-γ, IL-1β, and IL-6 production and an increased rate of survival upon challenge with LPS, suggesting that this enzyme is a key component in the inflammatory process and a potential target for anti-inflammatory therapy (3). Activation of MK2 results in the production of cytokines by regulating the translation and or stability of the encoding mRNAs through the AU-rich elements of the 3′-untranslated regions of the gene (4). MK2 also phosphorylates the transcription factor CREB, as well as leukocyte specific protein-1 and heat shock protein 25/27, which are involved in the regulation of actin polymerization (5–8) and cell migration (9, 10).
MK2 is a multi-domain protein consisting of an N-terminal proline-rich domain, a catalytic domain, an autoinhibitory domain and at the C-terminus a nuclear export signal (NES) and nuclear localization signal (NLS) (11–15). Two isoforms of human MK2 have been characterized. One isoform consists of 400 amino acids and the other isoform 370 residues which is thought to be a splice variant missing the C-terminal NLS (11, 16, 12). MK2 is located in the nucleus of the cell and upon binding and phosphorylation by p38, the MK2 NES becomes functional and both kinases are co-transported out of the nucleus to the cytoplasm (8, 12, 13, 17). Interestingly, transport of the MK2/p38 complex does not require catalytically active MK2, as the active site mutant, Asp207Ala, is still transported to the cytoplasm (13). Phosphorylation of human MK2 by p38 on residues T222, S272 and T334 is thought to activate the enzyme by inducing a conformational change of the autoinhibitory domain thus exposing the active site for substrate binding (8, 18). Mutations of two autoinhibitory domain residues W332A and K326E in murine MK2 demonstrate an increase in basal activity and a C-terminal deletion of the autoinhibitory domain renders the enzyme constitutively active, providing additional evidence to the role of this domain in inhibition of MK2 activity (18).
Recently, Meng, et al., published the structure of the autoinhibited, inactive form of MK2 47-400. However, since MK2 47-400 used by Meng, et al. included the autoinhibitory domain and was otherwise inactive, that structure is less useful for drug design.